1. Technical Field
The present invention relates to a sequence assignment method in mobile communication systems. More particularly, the present invention relates to a sequence assignment method of assigning a CAZAC (Constant Amplitude Zero Auto-Correlation) sequence to cells in a case of using a CAZAC sequence for an uplink reference signal.
2. Description of the Related Art
In 3GPP LTE (3rd Generation Partnership Project Long-term Evolution), studies are underway to use a CAZAC sequence such as a Zadoff-Chu sequence (hereinafter simply “ZC sequence”) and a generalized chirp-like sequence (hereinafter simply “GCL sequence”) as an uplink reference signal. A ZC sequence, which has especially good cross-correlation characteristics among CAZAC sequences, becomes a focus of attention.
A ZC sequence is represented by the following equation 1. Here, N is the sequence length and r is the sequence number. N and r are coprime.
                    (                  Equation          ⁢                                          ⁢          1                )                                                                                  c            r                    ⁡                      (            k            )                          =                  {                                                                                          exp                    ⁡                                          (                                              j                        ⁢                                                                              2                            ⁢                            π                            ⁢                                                                                                                  ⁢                            r                                                    N                                                ⁢                                                  (                                                      k                            +                                                                                          k                                2                                                            2                                                                                )                                                                    )                                                                                                            when                    ⁢                                                                                  ⁢                    N                    ⁢                                                                                  ⁢                    is                    ⁢                                                                                  ⁢                    even                                                                                                                    exp                    ⁡                                          (                                              j                        ⁢                                                                              2                            ⁢                            π                            ⁢                                                                                                                  ⁢                            r                                                    N                                                ⁢                                                  (                                                      k                            +                                                          k                              ⁢                                                                                                k                                  +                                  1                                                                2                                                                                                              )                                                                    )                                                                                                            when                    ⁢                                                                                  ⁢                    N                    ⁢                                                                                  ⁢                    is                    ⁢                                                                                  ⁢                    odd                                                                        ,                                                  ⁢                          k              =              0                        ,            1            ,            …            ⁢                                                  ,                          N              -              1                                                          [        1        ]            
With regards to ZC sequence, if the sequence length N is a prime number, N−1 sequences of good cross-correlation characteristics can be generated. At this time, the cross-correlation between sequences is √{square root over (N)} and fixed. For example, the cross-correlation between the ZC sequence of sequence number r=1 and the ZC sequence of sequence number r=5 that are different sequence numbers, is √{square root over (N)} and fixed. On the other hand, if the sequence length N is a number other than prime numbers and if the absolute value of the difference between two sequence numbers is not coprime to N, the maximum value of cross-correlation between these two sequences is greater than √{square root over (N)}.
Further, with regards to a ZC sequence, it is possible to generate, from one sequence, a plurality of cyclic shift sequences shifted m times the amount of cyclic shift 6 (hereinafter the cyclic shift sequence number is m.) Then, autocorrelation is ideally zero in all cyclic shift sequences other than the cyclic shift sequence, which the amount of shift is zero.
Here, the amount of cyclic shift Δ is designed based on delay spread. This is because, if delay spread exceeds the amount of cyclic shift Δ, a detection error occurs in the base station by detecting a correlation value between cyclic shift sequences of different amounts of cyclic shifts within the range of a delay profile detection window set in association with m times the amount of cyclic shift Δ.
Further, the transmission time length L of a reference signal is set to a given length, and therefore the number of cyclic shift sequences M that can be generated from one ZC sequence is limited, and the following equation 2 holds. [2]M=L/Δ  (Equation 2)
As a method of assigning a ZC sequence to cells in cases where a ZC sequence having these characteristics is used as a reference signal, studies are conducted for assigning different ZC sequences to neighboring cells and assigning M cyclic shift sequences that can be generated from one ZC sequence in a cell (see Non-Patent Document 1). FIG. 1 shows this conventional assignment method. Here, ZC sequences of different sequence numbers (r=1 to 21) are assigned to cells, and cyclic shift sequences (m=1 to 6) are used in each cell. Here, each cell corresponds to a radio service area. That is, in cases where one base station provides communication service for a plurality of radio service areas, a plurality of cells belong to one base station. When a plurality of cells belong to one base station, a plurality of cells each may be referred to as a “sector.” The same applies to the following explanation.
In this way, by assigning ZC sequences having low cross-correlations and having different sequence numbers to neighboring cells, it is possible to reduce inter-cell interference. Further, by using cyclic shift sequences having good autocorrelation characteristics in the same cell, it is possible to reduce interference between mobile stations in the cell and perform accurate channel estimation. The effect of reducing interference by a plurality of cyclic shift sequences generated from one ZC sequence is greater than by a plurality of cyclic shift sequences of different sequence numbers.
Non-patent Document 1: R1-062072, MOTOROLA, “Uplink Reference Signal Multiplexing Structures for E-UTRA,” 3GPP TSG RAN WG1 Meeting #46, Aug. 28-Sep. 1, 2006